Afghan civilians continue to bear the brunt of the country’s violence and United Nations estimates suggest the toll is rising despite Nato claims of improving security. Insurgent bombs, shootings and executions account for at least three quarters of civilians killed during the conflict, the UN says.

Sidiq Sidiqqi, spokesman for the Interior Ministry, said: “Unfortunately a suicide attack happened at around 8.40am as there was a joint patrol in the city. This particular area, called Mujahid Chowk, is particularly busy.”

A statement from coalition headquarters in Kabul confirmed three service members and an interpreter died in Eastern Afghanistan, but did not say where the attack happened.

Investigators were meanwhile still trying to determine what happened in a confused weekend clash between American and Afghan allies in Wardak province which left an American soldier and contractor dead and three Afghan soldiers dead.